Opacity monitors are used in facilities wherein exhaust gasses from the combustion of carbon-based fuels can release pollutants into the atmosphere through the facility's chimney. Generally, such opacity monitors utilize an optical detection system that measures opaqueness in the stream of exhaust gasses across the chimney or across an exhaust duct leading to the chimney. The opacity monitor creates an electronic signal that is proportionate, either directly or inversely, to the opaqueness detected in the exhaust stream. This electronic signal is used for automatic or manual control of the facility and the combustion process and for other facility operations or reporting purposes.
Opacity monitors do not have the ability to measure or indicate the appearance of the emission of exhaust gasses from a facility burning carbon-based fuels once such gasses have left the facility's chimney. Opacity, as measured by an opacity monitor, and the curbside appearance of the visual emissions from the chimney can diverge greatly. Combustion quality is one of the largest contributors to the appearance of chimney emissions. When carbon-based fuels are not completely burned because of a lack of the presence of sufficient oxygen during the combustion process, carbon is exported in the exhaust stream out of the chimney. This condition causes high opaqueness of the exhaust stream whereby it is very visible with a black to dark brown color resulting in a high opacity reading from the opacity monitor. This incomplete burning creates a situation where the correlation between the curbside appearance of exhaust gasses and the measured opacity is accurate.
Excess oxygen also affects the combustion process. The presence of excess oxygen beyond what is necessary for complete combustion of the fuel results in a plume being emitted from the facility's chimney. A plume is a visible emission from a facility's chimney that is light gray to white in color that can be very dense or highly opaque under certain conditions. When this heavy plume is present the correlation between the curbside appearance and the measured opacity is poor. The opacity monitor is unable to detect the plume because its formation occurs further down the exhaust stream than where the opacity monitoring device is located. In fact, the plume is generally formed in the atmosphere after the exhaust gasses leave the chimney.
Previous attempts to detect the presence of a visible plume have involved the use of optical detection devices to signal the presence of a smoke plume. U.S. Pat. No. 4,320,975 (Lilienfield), for example, involves a device that operates by measuring the proportion of polarized blue light from the background sky which passes through the plume. Unlike the present invention; however, Lilienfield requires the mounting of a device to “look” through the plume, which may be affected by environmental conditions as well as creating maintenance problems. Moreover, Lilienfield fails to address accuracy issues that may be caused by ambient conditions such as nighttime, cloudy days, etc.
An unsatisfied need therefore exists for systems and methods to determine the presence of an exhaust plume so that such detection can create an electronic signal for the automatic or manual control of the combustion process and to better comply with the United States Environmental Protection Agency's (“EPA's”) regulations and guidelines and with other clean-air laws. The opacity monitor is used for this combustion control process but it is overridden when the presence of an exhaust plume is indicated because the two signals (opacity and plume presence) require opposite control action of the same combustion process control variable, excess air. The plume presence signal is developed using combustion related control variable measurements that may be available to a facility's distributed control system (“DCS”).